An α+β titanium alloy has been in use for a long time as members of airplanes etc., with its high specific strength utilized. These days, the weight ratio of the titanium alloy used for airplanes is increasing, and the importance thereof is becoming higher and higher. Also in the consumer product field, an α+β titanium alloy having a high Young's modulus and a light specific gravity is increasingly used for golf club faces. In particular, in this use, since a thin sheet is used as the material in many cases, the need for a high-strength α+β titanium alloy thin sheet is high. Furthermore, a high-strength α+β titanium alloy is expected to be used also in automobile parts etc. in which weight reduction is regarded as important, and the need for a thin sheet, centering on a cold-rolled and annealed sheet, is increasing also in this field.
In the use of golf club faces, it is known that, when the direction in which a high strength and a high Young's modulus are exhibited in the sheet plane is used for the side of the short side of the face, the rebound regulation can be met and the durability is high. In this regard, when an α+β titanium alloy is subjected to unidirectional hot rolling, a texture called a transverse-texture (T-texture), in which the c-axis of the α-phase is strongly oriented in the sheet width direction, is exhibited where the α-phase is the main phase and exhibits a hexagonal closed packed (HCP) structure. At this time, in the α+β titanium alloy, the twinning deformation is suppressed and the slip direction of the primary slip system, which determines the plastic deformation, is limited in the bottom plane, and therefore the strength in the sheet width direction is increased in the case of having a T-texture. Thus, the rebound regulation is met and the durability is improved by using the sheet width direction of a unidirectionally hot-rolled sheet for the side of the short side of the face.
Patent Literature 1 discloses an α+β titanium alloy sheet that, while utilizing this phenomenon to attempt T-texture development and the accompanying improvement in strength and Young's modulus in the sheet width direction, has chemical components that prevent excessive development of a texture and the accompanying excessive strength increase and ductility reduction. Further, also for automobile parts, Patent Literature 2 discloses an automobile engine component and a material thereof in which cutting processing is performed such that the sheet width direction of an α+β titanium alloy sheet having a T-texture coincides with the axial direction of an engine component such as an engine valve or a connecting rod and thereby the strength and rigidity in the axial direction are increased. Both the technologies utilize a T-texture produced in an α+β titanium alloy unidirectionally hot-rolled sheet. However, in both the alloys, the amount of added Al, which reduces the cold ductility, is high, and cold rolling is difficult; hence, both the technologies are technologies in unidirectionally hot-rolled sheets, and production technology for a cold-rolled sheet of a smaller sheet thickness, for example a sheet thickness of 2.5 mm or less, has not yet been revealed.
On the other hand, for the α+β titanium alloy, there are proposed some α+β titanium alloys that allow a cold-rolled sheet to be produced. In Patent Literature 3 and Patent Literature 4, low-alloy-based α+β titanium alloys in which Fe, O, and N are used as main additive elements are proposed. By adding Fe as a β-stabilizing element and O and N as α-stabilizing elements, which elements are inexpensive, and adding O and N in amounts in appropriate ranges and with an appropriate balance, a balance of high strength and high ductility can be ensured. Since high ductility is provided at room temperature, it is presumed that a cold-rolled product can be produced. Further, in Patent Literature 5, while Al, which contributes to increasing the strength but reduces the ductility and reduces the cold processability, is contained, Si and C, which are effective in strength increase and yet do not impair the cold ductility, are added; thus, cold rolling is enabled. In Patent Literature 6 to Patent Literature 10, technologies in which Fe and O are added to control the crystal orientation, the crystal grain size, etc. to improve the mechanical characteristics are disclosed.
Further, in Patent Literature 11, the texture that an α+β titanium alloy hot-rolled sheet should have in order to ensure high cold ductility is described, and a technology, in which the cold ductility and the coil treatability in cold working are improved when the hot-rolled sheet has a developed T-texture, is disclosed. Thus, it is presumed that the cold ductility of a titanium alloy hot-rolled sheet having chemical components and a texture described in Patent Literature 11 will be good and a thin cold-rolled product can be produced relatively easily. However, when the α+β titanium alloy described in Patent Literature 3 to Patent Literature 11 is cold rolled and then annealed, depending on the combination conditions of cold rolling and annealing, it is likely that a basal-texture (B-texture) in which the c-axis of HCP is orientated in a direction close to the direction normal to the sheet will be produced, and the T-texture produced by unidirectional hot rolling will be damaged; hence, it has been difficult to maintain a high strength and a high Young's modulus in the sheet width direction.